Integrated devices are usually created on substrates in the form of wafers, which are used mainly as support for the fabrication thereof. However, the increasing degree of integration and the increasing performance levels expected of these devices is driving an increasingly significant coupling between their performance levels and the characteristics of the substrate on which they are formed. That is particularly the case with radiofrequency (RF) devices, processing signals with a frequency of between approximately 3 kHz and 300 GHz, the applications of which notably fall within the field of telecommunications (cellular telephones, WI-FI®, BLUETOOTH®, etc.).
As an example of device/substrate coupling, the electromagnetic fields, deriving from the high-frequency signals propagating in the devices, penetrate into the depth of the substrate and interact with any charge carriers located therein. This causes problems of nonlinear distortion (harmonics) of the signal, a pointless consumption of a portion of the energy of the signal by insertion loss and possible influences between components.
Thus, the RF devices have characteristics governed both by their architecture and their creation processes, and by the capacity of the substrate on which they are fabricated to limit the insertion losses, the cross-talks between neighboring devices, and the nonlinear distortion phenomena generating harmonics.
The radiofrequency devices, such as antenna switches, tuners and power amplifiers, can be created on different types of substrates.
Silicon-on-sapphire substrates are, for example, known, commonly called SOS (silicon-on-sapphire), which give the components, created according to microelectronic technologies in the surface layer of silicon, the benefit of the insulating properties of the sapphire substrate. For example, the antenna switches and power amplifiers fabricated on this type of substrate exhibit very good figures of merit but are primarily used for niche applications because of the overall cost of the solution.
Also known are the substrates based on high-resistivity silicon comprising a support substrate, a trapping layer arranged on the support substrate, a dielectric layer arranged on the trapping layer, and an active semiconductive layer arranged on the dielectric layer. The support substrate usually exhibits a resistivity higher than 1 k ohm·cm. The trapping layer can comprise non-doped polycrystalline silicon. The combination of a high-resistivity support substrate and of a trapping layer according to the prior art makes it possible to reduce the above-mentioned device/substrate coupling and thus ensure good performance levels in the RF devices. In this respect, a person skilled in the art will find a review of the performance levels of the RF devices fabricated on the high-resistivity semiconductive substrate known from the prior art in “Silicon-on-insulator (SOI) Technology, manufacture and applications,” points 10.7 and 10.8, Oleg Kononchuk and Bich-Yen Nguyen, from Woodhead Publishing.
Nevertheless, a trapping layer of polysilicon presents the drawback of undergoing a partial recrystallization in high-temperature heat treatment steps, which contributes to diminishing the trap density in the layer. With the trend in mobile telephone standards dictating increasingly demanding specifications in the RF components, the degradation of the performance of the device linked to this decrease in trap density is prohibitive for some applications.
Moreover, the step of deposition of the polysilicon and of surface preparation in order to produce the stacking of the substrate are sensitive and expensive.
An alternative to this trapping layer of polysilicon is a layer of porous silicon. A deposition of a porous layer, according to the prior art, does not make it possible to obtain a very thin layer thickness, less than 1 μm. Thus, the porous layers of the prior art and their thickness, between 10 μm and 80 μm, do not make it possible to obtain a substrate comprising a porous layer with a mechanical strength that is sufficient to withstand certain steps of fabrication of the devices and be retained in the final functional devices.